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Ages ago, as land plants were evolving, they ran into a few impediments. Soil can sometimes prove a nutrient-poor and inhospitable environment. In order to grow and thrive, plants need nitrogen to make proteins, but they lack the chemistry set to convert free nitrogen in the air into a form their cells can use.

To overcome these obstacles, early plants struck deals with co-evolving bacteria and fungi.

Some early bacteria developed the chemical tools to harvest free nitrogen from the air and convert it into forms such as ammonia and nitrate through a process called nitrogen fixation. The catch is that they need sufficient amounts of energy in the form of carbohydrates to power these conversions, and the supply of carbohydrates in the soil can be limited.

On the other hand, plants produce copious amounts of carbohydrates as a product of photosynthesis. And ammonia and nitrate are perfect protein-building nitrogen forms for plants.

Fungi and Bacteria: The Fundamental Fertilizers

Ages ago, as land plants were evolving, they ran into a few impediments. Soil can sometimes prove a nutrient-poor and inhospitable environment. In order to grow and thrive, plants need nitrogen to make proteins, but they lack the chemistry set to convert free nitrogen in the air into a form their cells can use.

To overcome these obstacles, early plants struck deals with co-evolving bacteria and fungi.

Some early bacteria developed the chemical tools to harvest free nitrogen from the air and convert it into forms such as ammonia and nitrate through a process called nitrogen fixation. The catch is that they need sufficient amounts of energy in the form of carbohydrates to power these conversions, and the supply of carbohydrates in the soil can be limited.

On the other hand, plants produce copious amounts of carbohydrates as a product of photosynthesis. And ammonia and nitrate are perfect protein-building nitrogen forms for plants.

A group of enterprising plants called the legumes (these include all beans and peas as well as clover and alfalfa) entered into a merger agreement with nitrogen-fixing bacteria called Rhizobium.

The bacteria moved into the plants’ roots, forming bumps on the roots called nodules that supply the fixed nitrogen plants need. In exchange, the plants supply the bacteria with the carbohydrates they require. Because Rhizobia can still dwell independently in the soil, plants are more dependent upon them than the microbes are on plants.

Animals need nitrogen for protein building, too. We humans get our nitrogen by eating plants (or by eating animals that eat plants). Another partnership teams plants with soil-dwelling fungi called mycorrhizae. Virtually all plants from flowers to towering trees like Sequoias have partner mycorrhizae.

Some species of mycorrhizae cover the surface of plants’ root hairs; others settle down inside the plant roots. The fungi act as extensions of the plants’ roots, vastly increasing the surface space of their nutrient-absorbing network.

Mycorrhizae increase the plants’ uptake of water and essential nutrients, particularly phosphorous, which doesn’tspread readily in soil. Inexchange, the plants provide the fungi energy in the form of carbohydrates.

This partnership enables both plants and fungi to survive in nutrient-poor places where they otherwise might die.